What is the difference in SAT II Math?

There are two levels: 1C and 2C. 1C covers topics usually taught in the first three years of high school (though in my experience it was more like middle school), whereas 2C also covers trigonometry and pre-calculus. If you want to give your friend some advice, I'd say tell his child to take 2C. First, unless the kid is sure that he will have little to do with either science or math in the future, it's best to take 2C. Secondly, 2C, despite covering more material, is actually easier to get a higher score on. This is because the SAT IIs are scored on a curve scale. To explain this, let's suppose there are 50 questions on both tests and 10% of the people who take 1C get all the questions right, whereas only 5% who take the 2C get all the questions right. The SAT creators will have decided beforehand the percentage of people who can get an 800--let's suppose that's 10% for all tests. Then, in order to get an 800 in the 1C, you'd have to get all the questions right, whereas on 2C it's possible to miss a few. Now, these tests aren't hard in terms of the math needed, but it's quite possible to make careless errors here and there, contributing to a not-as-perfect score. The 1C does not bode well for those who tend to make careless errors (I have heard the current scale definitely requires you to get all questions right in order to get an 800). In general, it's easier to obtain a higher score all the while getting fewer answers right on the 2C. Also, there's no need to take more than one test--the colleges only really want to see one score. It'll also be difficult to study for two tests (especially because the studying methods are different) when he could put all his effort into one test and ace it.

As far as I know, you can take as many SATII's as you want. Some colleges, though, will only want scores from two or three of your SATIIs. Anyway, the differences in the two levels of SATII Math is in their subject matter. The student should just ask their math teacher which SATII they should take. That's what I did back in the days; I just asked my math teacher which math SATII I should sign up to take, based on her class's subject matter. (on a side note, it was a trigonometry class; don't take my word for this, but I think the other math SATII covers precalculus or something).